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Road traffic emissions are often considered the main source of ultrafineparticles (UFP, diameter smaller than 100 nm) in urban environments.However, recent studies worldwide have shown that – in high-insolation urbanregions at least – new particle formation events can also contribute to UFP.In order to quantify such events we systematically studied three citieslocated in predominantly sunny environments: Barcelona (Spain), Madrid(Spain) and Brisbane (Australia). Three long-term data sets (1–2 years) offine and ultrafine particle number size distributions (measured by SMPS,Scanning Mobility Particle Sizer) were analysed. Compared to total particlenumber concentrations, aerosol size distributions offer far more informationon the type, origin and atmospheric evolution of the particles. By applying<i>k</i>-means clustering analysis, we categorized the collected aerosol sizedistributions into three main categories: "Traffic" (prevailing 44–63% ofthe time), "Nucleation" (14–19%) and "Background pollution andSpecific cases" (7–22%). Measurements from Rome (Italy) and Los Angeles(USA) were also included to complement the study. The daily variationof the average UFP concentrations for a typical nucleation day at each siterevealed a similar pattern for all cities, with three distinct particlebursts. A morning and an evening spike reflected traffic rush hours, whereasa third one at midday showed nucleation events. The photochemicallynucleated particles' burst lasted 1–4 h, reaching sizes of 30–40 nm. Onaverage, the occurrence of particle size spectra dominated by nucleationevents was 16% of the time, showing the importance of this process as asource of UFP in urban environments exposed to high solar radiation. Nucleation events lasting for 2 h or more occurred on 55% ofthe days, this extending to > 4 h in 28% of the days,demonstrating that atmospheric conditions in urban environments are notfavourable to the growth of photochemically nucleated particles. In summary,although traffic remains the main source of UFP in urban areas, in developedcountries with high insolation urban nucleation events are also a mainsource of UFP. If traffic-related particle concentrations are reduced in thefuture, nucleation events will likely increase in urban areas, due to thereduced urban condensation sinks

Traffic and nucleation events as main sources of ultrafine particles in high-insolation developed world cities